Pedicular screw and posterior spinal instrumentation

ABSTRACT

The instrumentation comprises for each lumbar vertebra two pedicular screws having two screw threads, one of the screw threads corresponding to the region of penetration of the screw in the pedicle, two rods having asperities respectively associated with a pedicular screw and secured to the latter by connection elements provided with elements for clamping the rods. Each connection element is in one piece and the screw thread corresponding to the region of penetration of the screw in the pedicle terminates in an annular shoulder against which the connection element bears which is separated by a smooth part from the screw thread outside the region of penetration in the pedicle. This instrumentation permits both returning the slipped vertebrae rearwardly and pivoting it in the desired direction of rotation, it being possible to carry out the derotation action slowly and by controlling the movement so that the surgeon achieves on the whole in the two movements an excellent precision in the repositioning of vertebrae relative to the neighboring vertebrae.

The invention relates to lumbar osteosynthesis instrumentation for thecorrection of spondylolisthesis.

It is known that, substantially, spondylolisthesis is the forwarddisplacement of a vertebra relative to its lower neighbour. In theory,any vertebra may be affected, but the fifth and the fourth lumbarvertebrae are the most commonly concerned.

Affecting more frequently women or young girls than men,spondylolisthesis is usually classified into five types: dysplastic,isthmian, traumatic, degenerative and pathological.

Its degree of seriousness is measured by the distance travelled throughby the displaced vertebra with respect to its lower neighbour.

There are four stages:

The first stage is a displacement of a quarter of the antero-posteriordiameter of the vertebral body.

The fourth stage corresponds to a complete displacement of the vertebralbody.

The second and third stages are the intermediate stages.

Heretofore, although there are certain surgical techniques for thetreatment of spondylolisthesis, either by the direct traction on theslipped vertebra, or by the screwing of the pedicles of this slippedvertebra, the reduction of the spondylolisthesis is not alwayssatisfactory, above all in the cases of serious spondylolisthesis.

Surgery is indeed indicated for those who have a long past of lumbalgiaor sciatica, in the case of evidence of a vertebral canal stenosis, acompression of the cauda equina or a subjacent motor lesion, or thosewhose spondylolisthesis rapidly evolves toward stage 3 or stage 4.

Generally, in surgery, an anterior or posterior vertebral fusion,laminectomy with decompression of the posterior structures and anexcision of the hypertrophied mass of the fibrous tissue in the regionof the lysis may be indicated.

In isthmian spondylolisthesis, the bilateral isthmian lyses are liableto be associated with a considerable pseudoarthrosis in relation with anemerging nerve root. The discal state in the region of thespondylolisthesis sometimes requires a radiculographic or discographicassessment. In vertebral canal stenoses, the decompression withoutfusion may be a surgical operation.

In actual fact, an effective orthopedic surgery consists in returningthe vertebra not only onto the axis of the spine but also to a positionwhich is as correct as possible relative to the neighbouring vertebrae.Over a period of time, or in the course of its displacement, thisvertebra in fact might have been subjected to lateral thrusts which havecaused it to pivot horizontally to a more or less large extent so that acorrect repositioning of the vertebra of course implies its rearwardreturn but also a derotation.

Attempts have already been made to correct spondylolisthesis, forexample by means of devices consisting of two pedicular screwsinterconnected by a transverse plate which, by screwing a nut bearingagainst the plate, act solely by translation of the concerned vertebrafor putting it into alignment with the neighbouring vertebrae.

Thus, the device disclosed in French patent 2 615 095 (87 06 864)employs two rods which are longitudinally fixed in the vertebral columnwith the aid of pedicular screws and each serve as support means for twoscrews having a double screw thread connected to the vertebra to becorrected. These two double thread screws are transversely connected bya rigid plate constituting a bipedicular base. It is this transverseplate which permits acting on the vertebra to be corrected owing to theprovision of a median opening for receiving traction forceps.

In fact, such a device does not permit acting in a sufficientlysatisfactory manner, and experience has shown that a traction on themedian part of the plate has for effect to rearwardly return thevertebra to be corrected too rapidly. Further, if this vertebra must bederotated, it has been found that the presence of the plate is liable toprevent the required derotation action owing to the fact that thisinduces at the same time a certain return of the vertebra.

Now, surgical experience has revealed that it is desirable to act slowlyon the vertebra by acting on each one of the double thread screwsindependently, so that the surgeon can gradually adjust for eachparticular case his vertebra derotation and/or return action.

Moreover, such a device is relatively complex and costly owing to thenumber of its component parts which further increases the mountingdifficulties encountered by the surgeon during the surgical operation.As concerns in particular the double thread screw, in addition to thefact that it is long and costly to produce, its implantation in thepedicles is not at all convenient.

Further, this double thread screw is subjected to extremely high shearstresses, mainly in the region of the junction with the instrumentationappliances or in the upper part of the screw thread.

An object of the invention is to provide a lumbar osteosynthesisinstrumentation for the correction of spondylolisthesis, which permitsboth returning the vertebra to the rear and pivoting it in the desireddirection of rotation. Another object of the invention is to provide adouble thread pedicular screw so arranged as to overcome theaforementioned drawbacks while permitting mass production which ischeaper than in the case of screws known in the art which can only beproduced on a small scale.

The osteosynthesis instrumentation according to the invention comprises,for each lumbar vertebra, two pedicular double thread screws, one threadof which corresponds to the region of penetration of the screw in thepedicle, two rods having surface asperities respectively associated witha pedicular screw and connected to the latter by connection elementsprovided with elements for clamping the rods.

According to the invention, each connection element is in one piece andthe screw thread corresponding to the region of penetration of the screwin the pedicle terminates on an annular shoulder on which the connectionelement bears.

The derotation action exerted by means of this instrumentation has theadvantage that it can be carried out slowly and with a certain amount ofcontrol over the movements, thereby affording on the whole in the twomovements an excellent precision in the repositioning of the vertebrarelative to its neighbouring vertebrae.

Further, the arrangement of a transverse shoulder into whichtangentially fades the screw thread of the part of the screw serving topenetrate the pedicle, provides a reinforcement or strengtheningresisting the shear stresses in this region. The continuous peripheralshoulder may also act as a support for an added part.

According to a particular feature of the invention, the end of the screwthread outside the part of the screw serving to penetrate the pedicle isextended by a screw threaded rod adapted to cooperate with an ancillaryscrewing device.

The instrumentation according to the invention comprises an ancillarydevice for screwing the pedicular double thread screw, this devicebearing against the screw threaded extension of said screw. Thisancillary device comprises a rod provided with an end grip and includesa tubular end part which is arranged to permit the insertion of theoperating part of the double thread screw, has a conical free end partremote from the grip tapering toward the grip and is provided with atleast one longitudinal slot. This ancillary device further comprises atube freely slidable on the rod for the purpose of surrounding andgripping the conical end part of its tubular end part for the purpose ofclamping it on the operating part of the pedicular screw.

With this ancillary device, the length of the operating screw thread isalmost completely enclosed in the tubular end part and clamped owing tothe action of the second tube when the latter surrounds the conical endpart of the tubular end part. After having provided a prior bore in thepedicle, the surgeon can in this way easily screw the double threadscrew in position.

Further features and advantages of the invention will be apparent fromthe following description with reference to the accompanying drawingswhich illustrate two embodiments of the invention by way ofnon-limitative examples.

In the drawings:

FIG. 1 is a perspective view from above of one embodiment of the lumbarosteosynthesis instrumentation according to the invention, mounted onthe first lumbar vertebra and on the sacrum.

FIG. 2 is a partial perspective view of the instrumentation shown inFIG. 1.

FIG. 3 is an elevational view in the transverse direction, partly insection, of the instrumentation shown in FIGS. 1 and 2.

FIG. 4 is an elevational view to a larger scale of a pedicular doublethread screw which is part of the instrumentation shown in FIGS. 1 to 3.

FIG. 5 is a perspective view of an embodiment of the ancillary devicefor mounting the pedicular screw shown in FIG. 4.

FIG. 6 is a perspective view to a larger scale of an alternativeembodiment of the connection element between the osteosynthesis rod andthe screw.

FIG. 7 is a sectional view taken on line 7--7 of FIG. 6.

The lumbar osteosynthesis instrumentation shown in FIGS. 1 to 3 isadapted to correct spondylolisthesis by the posterior approach.

It comprises, for each lumbar vertebra to be corrected, for example thevertebra L5, two pedicular screws 1 having two screw threads, twocylindrical rods 2, preferably two rods of the so-called Cotrel typehaving surface asperities or knurling extending longitudinally along theconcerned lumbar segment (vertebrae L5, L4 and sacrum S in the assemblyshown in FIG. 1), connection elements 3 connecting each pedicular screw1 to the adjacent rod 2, pedicular screws 4 for securing the rods 2 tothe vertebra L4 and to the sacrum S, and devices 5 providing atransverse connection between the rods. All these elements except thepedicular screws 1 and the connection elements 3 are known per se andtherefore need no special description.

Each connection element 3 has, when viewed in the transverse direction,a substantially L-shaped profile consisting of an enlarged base 3a inwhich is provided a through passage 10 receiving the rod 2 and having anaxis X--X. This portion 3a has on one side a cylindrical surface and onthe other side a planar surface and is extended by a thin portion 3b.The latter has two parallel planar surfaces and is provided with anopening 7 for the passage of the pedicular screw 1. The opening 7consists of a cylindrical central part having an axis Z--Z extended oneach side by two conical parts 7b, 7c opening onto the planar surfaces.The taper of these conical parts is such that it permits an inclinationof the axis of the screw 11 relative to the axis Z--Z of about ±15°, theangular limits of this axis corresponding to the axes Y--Y and Y'--Y'(FIG. 7). A tapped opening 6 transversely opens onto the bore or passage10 and receives a screw 20 for radially clamping the rod 2. The surface3c of the element 3 remote from the passage 10 is completely planar.

The element 3 constitutes the means for securing the rod 2 and the meansfor screwing the screw 1. The distance between the axes of the screw 1and rod 2 vary with the subject so that the surgeon can choose the mostappropriate element 3.

Each screw 1 comprises from its point 8 a first screw threaded part 9extending to the length of penetration of the screw 1 in the pedicularand a second screw threaded part 11 which is outside the region ofpenetration in the pedicle and is termed the "operating" part. On theside remote from the screw threaded part 9 the operating part 11 isextended by a profiled end portion 12, constituted for example by twoflats 13 adapted to permit both the gripping of the double thread screw1 and the screwing thereof by means of an ancillary device 14 (FIG. 5)which will be described hereinafter.

At the end remote from the profiled end portion 12, the screw threadedoperating part 11 is extended by a short smooth region 15 terminating ina continuous peripheral shoulder 16 constituting the end of the screwthreaded part 9. This shoulder 16 bears against the planar surface ofthe element 3 which is the most remote from the screw threaded part 11,consequently the screw 1 cannot pass through the element 3.

The screw thread 18 of this screw threaded part is adjacent the shoulder16 and the depth of the thread 21 close to the shoulder 16 graduallydiminishes until it tangentially merges with or fades onto a cylindricalsector 22 bordered by the shoulder 16 which thus forms a screwreinforcing region in this position. The shoulder 16 may act as asupport for a device or an associated connection element 3.

The operating part 11 of the screw 1 is constituted by a metal screwthread of small pitch, for example between 0.5 and 2 mm and preferablybetween 0.8 and 1 mm, adapted to receive a nut 23 for clamping theconnection element 3 against the shoulder 16.

This instrumentation has the following advantages:

The fact that the connection element 3 is made in a single piece notonly makes this element cheaper to manufacture but also simplifies thepositioning of the element by the surgeon, and above all improves thestrength of the connection between the rod 2 having asperities and thepedicular screw 1.

As previously explained, the pedicular double thread screw 1 issubjected to high mechanical stresses of multiple origins; it mustresist these various stresses for a relatively long period of time theduration of which depends on each patient. The screw 1 must therefore beas simple as possible to mount during the surgical operations, but mustalso have sufficient strength to withstand all the forces during andafter the operation. Now, with the pedicular screws employed heretofore,it has been found that fractures generally occur in the region betweenthe two screw threads. The invention remedies this situation byterminating the screw thread 18 for anchoring in the pedicle in thesubstantially tangential region 22. Consequently, the screw thread 18does not "open out" onto the shoulder 16 for supporting the connectionelement 3 and consequently ensures that this shoulder 16 has aperipheral continuity and a sufficient supporting thickness. Thissupporting thickness thus affords the maximum mechanical resistance tothe forces transmitted to the connection element 3 and therefore exertedby the latter.

Another advantage afforded by this arrangement of the anchoring screwthread 18 resides in the fact that, when it is desired to effect abicortical anchorage by reaching the opposite cortical part of thevertebra and with the screw thread length corresponding to the length ofthe passage through the vertebra, the surgeon is informed of the desiredfinal position of the screw thread. The stoppage of the latter in itsupper part indeed constitutes a warning in that the surgeon mustsuddenly exert a higher torque. He must therefore stop his screwingeffort.

Owing to the provision of the profiled end portion 12, the engagement ofthe pedicular screw 1 by the ancillary device 14 no longer occurs in thecentral region of the screw, as described in said patent 2 615 095, butat the free end of the screw. This end region may be in the form of twoflats 13 provided on a smooth, or optionally screw threaded, end part topermit the ancillary device 14 to act on the screw.

This ancillary device comprises a rod 24 provided with a tubular endpart 25 and, at its opposite end, a manual grip 26. The tubular part 25is so dimensioned as to permit the insertion therein of the operatingpart 11 of the screw 1, and the ancillary device 14 is completed by atube or sleeve 27 coaxial with the rod 24 and slidably mounted on thelatter so as to be capable of surrounding the tubular part 25 at the endof its travel.

The tubular part 25 is provided with longitudinal slots 28 starting atits free end, for example three slots as illustrated, extending along apart of the length of the region 25. The free end portion 29 of thelatter is conical and tapers in the direction toward the grip 26. Beyondthe slots 28, at a certain distance from the latter, there is provided ahollow profile matching the profiled end portion 12 of the screw, forexample, as shown in FIG. 5, two flats 31 whose longitudinal surfacesare parallel. The two flats 31 are adapted to receive correspondingflats 13 of the screw 1 and thus lock the latter against rotation afterthe insertion of the operating part 11 in the tubular part 25. In thedirection toward the conical end part 29, the flats 31 are extended bylongitudinal recesses 32 for receiving the operating part 11 of thescrew 1. Lastly, in order to permit a visual checking of the position ofthe profiled end portion 12 on the flats 31, the tubular part 25 isprovided with a transverse opening 33 in the region of the flats.

When the operating part 11 is inserted in the tubular part 25 with itsend portion 12 locked against rotation between the flats 31, the surgeonslides the tube 27 along the tubular part 25 until the conical endportion 29 is made to grip round the operating part 11, the taperedportion 29 thus constituting a holding chuck for the screw 1. The twoflats constitute additional means for preventing a reverse rotation ofthe pedicular screw 1 in the ancillary device 14. The ancillary device14 also serves to screw the nut 23 clamping the screw 1 in theconnection element 3. It is provided in its end 29 with a correspondingrecess for engaging this nut 23.

The instrumentation just described is used in the following manner.

Before placing in position the support rods 2, which are for example theknown Cotrel rods (Registered Trademark), the surgeon suitably anchorswith his ancillary device 14 the double thread screws 1 in each of thepedicles of the vertebra L5 to correct. Then he secures the support rods2 to the neighbouring vertebrae, or, as in the embodiment shown in FIG.1, to the sacrum S and vertebra L4. Then he firmly fixes the connectionelements 3 to the rods 2 by means of a radial clamping screw 20 (FIG.1).

At this stage, the two transverse connection rods 5 are not yetinstalled. In order to correct the vertebra L5, i.e. to return itrearwardly (spondylolisthesis) and/or derotate it, the surgeon acts onthe nuts 23 clamping the connection elements 3 by means of an ancillarydevice and preferably two ancillary devices, each being disposed on oneof the nuts 23 of the two screws 1. He is now free to act, as desired,more on the screwing of one of the nuts 23 than on the screwing of theother, depending on the movement he wishes to impart to the vertebra L5,as he is free to act in an identical manner on the two nuts 23.

Each time the surgeon acts on one of the nuts 23, he exerts by thebearing of the screw thread-nut system on the connection element 3firmly connected to the support rod 2, a traction on the vertebra L5 ina direction toward the connection element 3. When he acts on a singleone of the nuts 23, he also produces a slight rotation of the vertebraL5 about itself. When he acts equally on both nuts 23, he produces arearward return of the vertebra L5.

In all cases, the fine pitch of the screw thread 11 supporting thecorrection nut 23 permits acting very progressively on the vertebra.When the vertebra is finally in the position required by the surgeon,the latter severs the double thread screw 1 above the nut 23. Thesevering of the rod deforms the screw thread 11 and this subsequentlyprevents the unscrewing of the nut 23.

It is only at this moment that the transverse rods 5 interconnecting therods 2 can be mounted, thereby achieving a rectangulation of the wholefinal instrumentation.

The instrumentation according to the invention is easier to place inposition by the surgeon than that disclosed in said French patent 2 615095, cheaper as concerns the fabrication of the screw, and more reliablein use; it no longer requires a transverse handling plate in the regionof the connection elements 3. The screwing of the pedicular screws 1 bytheir profiled end portions 12 rather than by their central regions asin the aforementioned prior instrumentation, is also more convenient forthe surgeon.

It must be understood that the scope of the invention is not intended tobe limited to the described embodiments and various modifications may bemade. For example, it is obvious that the end portion 12 may have anysuitable profile other than that described, the same being true of thecorresponding two flats 31 of the ancillary device 14.

Likewise, in the alternative embodiment shown in FIG. 6, the connectionelement 3 may be constructed to have, as viewed in the transversedirection, a substantially L-shaped profile and be disposed in suchmanner that its planar surface 3c (in contact with the nut 23 in FIG. 3)is inverted, by turning the element round, so as to be closer to theadjacent vertebra.

What is claimed is:
 1. A spinal fixation system, comprising:an elongated rod configured for attachment along a posterior aspect of a patient's spine, a set screw, and a nut; a pedicular screw having a first screw thread and a second screw thread, said first thread being configured for penetration into a posterior pedicular region of the patient's spine, said second thread being separated from said first thread by a bearing shoulder therebetween, said second thread being configured to extend outside the pedicular region when penetrated by said first thread; a connection element having a generally L-shaped profile defining a rod bore receiving said rod therethrough and a threaded bore intersecting said rod bore, said set screw being threaded into said threaded bore to correspondingly clamp said rod to said element, said element having two opposed planar surfaces defining an opening configured to receive said pedicular screw therethrough, said opening having a cylindrical central part extended at both ends by conical parts each opening out onto a respective one of said planar surfaces, said element bearing against said shoulder of said pedicular screw, said nut being threaded on said second thread of said pedicular screw fastening said pedicular screw to said element; and wherein said set screw is positioned anterior to said nut to provide a low profile construct when said rod and said pedicular screw are rigidly interconnected by said element.
 2. The system according to claim 1, wherein said connection element is defined by a single unitary piece having a generally flat, planar posterior surface without protrusions.
 3. The system according to claim 1, wherein said set screw is configured not to extend posterior to a generally flat, planar surface of said element when securely threaded into said threaded bore.
 4. The system according to claim 1, wherein said cylindrical central part and said conical parts of said opening permit selection of an angular inclination of said pedicular screw through said opening from a range of about 30°.
 5. The system according to claim 1, further comprising:an ancillary device for screwing said pedicular screw by an operating part thereof, said operating part extending outside the pedicular region of penetration of said pedicular screw, said ancillary device including a rod provided with an end grip and a tubular end part which is arranged to permit the insertion therein of said operating part of said pedicular screw, said tubular end part having a conical free end portion remote from said grip an tapering in a direction toward said grip and defining at least one longitudinal slot, and a tube freely slidably mounted on said rod of said device configured for surrounding said conical free end portion of said tubular end part to grip said operating part of said pedicular screw. 